1. Field of the Invention
The present invention relates to an improved process for the liquefaction of raw subdivided coal. More particularly, the invention relates to an improved liquefaction process wherein the coal is dissolved in a petroleum-derived solvent at an elevated temperature and hydrocracked at a lower temperature to produce acceptable fuel oils accompanied by minimum gas production.
2. Prior Art
Coal is our most abundant indigenous fossil fuel resource, and as a result of dwindling petroleum reserves, concerted research efforts are being directed towards recovery of liquid hydrocarbons from coal on a commercial scale. A promising approach in this field relates to the direct liquefaction of coal accompanied with minimum gas production. This approach has principally evolved from the early work of F. Bergius, who discovered that transportation fuels could be produced by the high-pressure hydrogenation of a paste of coal, solvent and catalyst. Later discoveries revealed the advantageous use of specific hydrogenation solvents at lower temperatures and pressures. With these solvents, such as partially saturated polycyclic aromatics, hydrogen is transferred from the solvent to the coal molecules, thus causing depolymerization and dissolution of the coal. The resulting coal liquid, however, has a high molecular weight and an accordingly high viscosity, which presents considerable obstacles to removing the fine coal residue remaining in the liquid, since these particles typically range in size from 1 to 25 microns in diameter. The complete nature of the coal residue, or undissolved solids, is not wholly understood; however, the residue appears to be a composite of organic and inorganic species. The residue organic matter is similar to coke, and the residue inorganic matter is representative of the well-known ash constituents. Removal of the residue from the coal liquid has been considered a critical step in the prior art in the preparation of clean fuels, particularly in those processes in which the coal liquids are subjected to catalytic upgrading, such as hydrocracking.
Nearly all crude petroleum stocks and especially crude residua contain metal compounds, but the amounts and types of metals may vary considerably depending upon the geographic and geological origin of the coal. Certain South America crudes, for example, contain large amounts of vanadium but only small amounts of other metals. Other crudes, for example Middle Eastern, contain a broad spectrum of metals in only moderate concentrations, mostly metals such as nickel and vanadium, and smaller amounts of such metals as iron and sodium. Still other crudes, for example California crudes, contain large amounts of many metals, including large amounts of iron and sodium. In the processing of petroleum feedstocks over fixed-bed hydrocracking catalysts to convert the higher-boiling fractions to lower-boiling fractions, it is generally recognized that a high metals content in the feed tends to rapidly foul the catalyst bed. The metal compounds present in the residua will form metalliferous deposits on and between catalysts in the bed and within the pores of the catalyst. The deposits on and between the catalyst particles may clog the catalyst bed, thereby restricting the oil throughput. Deposits within the catalyst pores result in an over-all deactivation of the catalyst. It is therefore an object of this invention to provide a process for the liquefaction of coal with a petroleum feedstock having a relatively high metals content whereby the liquefaction products may be directly hydrocracked without substantial fouling of the catalyst bed.